An extensive comparison of the various quantitative methodologies for estimating HDL cholesterol has been completed by utilizing the micromethod developed in our laboratory for the fractionation of lipoproteins and the quantitation of their cholesterol content. The results compare favorably with methods requiring a tenfold greater volume of plasma. The normal ranges for apoA-I and apoA-II have been measured by electroimmunoassay and radial immunodiffusion. Quantitation of these apolipoproteins by rate nephelometry is also being evaluated. Electroimmunoassays for apoC-I, C-III, D, and E are under active development.